Hydraulic fluid controls can be found in a variety of automotive applications such as automatic speed change transmissions as well as others. In these applications, it is often desirable to control the pressure of the hydraulic fluid, as seen by reference to U.S. Pat. No. 6,308,725 entitled “APPARATUS FOR CONTROLLING HYDRAULIC FLUID PRESSURE” issued to Lawlyes et al., assigned to the common assignee of the present invention. Lawlyes et al. disclose a smart actuator including a solenoid element and a pressure sensor element, both of which are in electrical communication with a remote control through a wire harness. Lawlyes et al. provide for remote pressure sensing of a solenoid output.
In the specific context of an automatic speed change power transmission, it is known to use electronic transmission control units that are configured to generate electrical signals that control solenoids resulting in the control of fluid flow as well as the pressure in a hydraulic fluid line. As known, the pressure in a hydraulic fluid line can be used to control various other elements in an automatic transmission system including for example a hydraulically-actuated clutch for the engagement of individual gears. By engaging various combinations of gears (e.g., planetary gears in a planetary gear transmission), an automatic transmission system accomplishes the same task as the shifting of gears in a manual transmission. Hydraulically-actuated clutches that are found in transmissions are typically used for engaging a pair of gears (e.g., a pair of rotating members, or alternatively, one rotating member and one non-rotating member) together such that when the clutch is applied, torque can be transmitted from one shaft to the other. Shift changes may also include switching three or more clutches on occasion for certain types of shifts, and herein references to two clutch type shifts could also include the multiple shifts.
An important operating aspect of a hydraulically operated clutch relates to the pressure build-up of the applied hydraulic fluid. In general, fluid flow at a certain applied pressure is sought to be controlled and varied to apply the clutch in order to obtain a desired engagement characteristic, principally with respect to timing and smoothness. It should be appreciated that if the timing of the engagement of one gear with the disengagement of another gear is not coordinately properly, overall shift performance may suffer. It is thus desirable and known in the art to control the clutch pressure.
In this regard, it is known to provide a linear solenoid to control the hydraulic fluid pressure to apply and/or release the clutch. In a linear solenoid, the amount of fluid at a controlled pressure can be varied by changing a solenoid control current. To achieve control of a system including a linear solenoid, it is known to employ software responsive to various inputs to control the current provided to the solenoid. It should be noted that typical hydraulically-actuated clutch arrangements that are fed by a linear solenoid (or are fed by a valve assembly containing a linear solenoid) are provided with either a very small hydraulic fluid accumulator or with no accumulator at all. This arrangement can be susceptible to pressure overshoot and undershoot. Notwithstanding the variety of factors that can cause pressure overshoot and/or undershoot in the pressure control system, a few solutions have been proposed in the art.
One conventional method to prevent or minimize overshoot and/or undershoot in a pressure control system of this type is to translate relatively large requested pressure changes into a series of smaller steps, thus limiting, in effect, the pressure change command in a stepwise fashion. However, such an approach results in a relatively slow response time and in any event may not be applicable to every configuration.
Another conventional method is to employ a hardware solution including integrated circuit (IC) technology using proportional-integral-derivative (PID) control. However, this solution requires determination of PID gains, which in turn depend on how much pressure prevails in the clutch, the fill amount and the like, all of which can result in control problems.
There is therefore a need for a hydraulic clutch pressure control system that minimizes or eliminates one or more of the problems set forth above.